1. Field of the Invention
The present invention relates to data communications. In particular, the present invention relates to ensuring compatibility on a high performance serial bus system.
2. The Prior Art
Background
Modern electronic equipment has greatly enhanced the quality of our lives. However, as the use of such equipment has increased, so has the need to connect equipment purchased from different manufacturers. For example, while a computer and a digital camera may each be useful when used alone, the ability to connect the digital camera to the computer and exchange information between the two makes the combination even more useful. Therefore, a need was apparent for a serial bus standard that would allow for the connection and communication between such devices.
The IEEE 1394-1995 standard was developed to satisfy this need. This standard revolutionized the consumer electronics industry by providing a serial bus management system that featured high speeds and the ability to “hot” connect equipment to the bus; that is, the ability to connect equipment without first turning off the existing connected equipment. Since its adoption, the IEEE 1394-1995 standard has begun to see acceptance in the marketplace with many major electronics and computer manufacturers providing IEEE 1394-1995 connections on equipment that they sell.
However, as technologies improved, the need to update the IEEE 1394-1995 standard became apparent. Two new standards are being proposed at the time of the filing of this application, herein referred to as the proposed IEEE 1394a, or P1394a standard, and the proposed IEEE 1394b, or P1394b standard. Improvements such as higher speeds and longer connection paths will be provided.
In the discussion that follows, it will be necessary to distinguish between the various standards that are being proposed as of the date of this application. Additionally, it will be necessary to distinguish hardware and packet transmissions that are compatible with the P1394b standard and not earlier standards.
Thus, the term “Legacy” will be used herein to refer to the IEEE 1394-1995 standard and all supplements thereof prior to the P1394b standard. Thus, for example, a Legacy node refers to a node compatible with the IEEE 1394-1995 standard and all supplements thereof up to, but not including, the P1394b standard.
Additionally, packets of data will be referred to herein depending on the context the packets are in. For example, a packet of data that is compatible with the P1394b standard and is traveling through a PITY compatible with the P1394b standard will be referred to as Beta format packets. Packets of data that are compatible with the Legacy standard but are traveling through a PHY compatible with the P1394b standard will be referred to as Legacy packets. Finally, packets of data that are compatible with the Legacy format and are traveling across a data strobe link will be referred to as Alpha format packets.
Furthermore, in the discussion that follows PHYs that are compatible with the P1394b standard may be referred to in various ways, depending upon the context the PHY is operating in and the capability of the PHY. For example, a PHY that has circuitry compatible with the P1394b standard but not any previous standards will be referred to as a B only PHY. Also, a PHY that is compatible with both the P1394b standard and all predecessors and is communicating with only devices compatible with the P1394b standard will be referred to as B PHYs. Finally, a PHY that is communicating with both Legacy devices and devices compatible with the P1394b standard will be referred to as a border device, border PHY, or border node.
Finally, a communications system that has only B PHYs attached will be referred to as a B bus.
Data Transmission in Legacy Systems
One area that has been improved in the P1394b standard is in the way that data transmission takes place on the bus.
FIG. 1 is a prior art example of a Alpha format data packet 100 according to Legacy specifications. In the IEEE 1394-1995 standard, a data packet will begin with the transmission of a Data Prefix (“DP”) identifier, shown as DP 102 in FIG. 1. Importantly, in the IEEE 1394-1995 standard, a DP must have a duration of no less than 140 nanoseconds (ns), though a DP may be of any greater length.
Typically, a DP is followed by the transmission of clocked data, known as the payload, shown as clocked data 104 in FIG. 1. On a Legacy bus, the payload will be clocked at a rate of 100 Megabits per second (Mb/s), 200 Mb/s, or 400 Mb/s. These data rates are known as S100, S200, and S400, respectively.
Finally, the payload is followed by a Data End (“DE”), shown as DE 106 in FIG. 1. In the IEEE 1394-1995 standard, a DE must be at least 240 ns in length.
As is appreciated by one of ordinary skill in the art, the Legacy specifications thus a timer-based system, where data transmission begins and ends according to a fixed timer.
Compatibility Issues in Legacy Systems
As mentioned above, there are three clocked data rates present in Legacy systems, S100, S200, and S400. Initially, when the IEEE 1394-1995 standard was introduced, devices could only communicate at the S100 rate. Later, devices were introduced that communicated at the S200 and S400 rates.
One problem that occurred in the prior art was how to insure compatibility between the various devices on the market that were communicating at these different rates.
FIG. 2 illustrates such a compatibility problem. FIG. 2 has three nodes, nodes #0, #1, and #2. Node #2, the root node in this example, wishes to communicate with node #1. As is indicated in FIG. 2, nodes #1 and #2 are capable of communicating at the S400 data rate, while node #0 is only capable of communication at the lower S100 rate.
FIG. 3 illustrates the prior art solution of speed filtering on a Legacy bus. FIG. 3 shows root node #2 transmitting S400 data in packet P1 to node #1. In the prior art, to prevent node #0 from receiving the S400 data that it cannot understand, it is “shielded” from such data by having root node #2 transmit a null packet P2 to it.
In the FIG. 3 Packet Detail illustration, packets P1 and P2 are shown together on a common time axis. Packet P1 comprises a DP 300, S400 data 302, and a DE 304. Null packet P2 comprises a DP 306, and a DE 308. As is appreciated by one of ordinary skill in the art, the null packet accomplishes its shielding by extending the DP for the amount of time required to send S400 data 302. As is known by those of ordinary skill in the art, on a Legacy bus all nodes must remain synchronized in their interpretation of idle time. Thus, the null packet effectively ‘busies’ node #0 while root node #2 transmits S400 data to node #1 and thus shields node #0 from speeds it cannot understand.
Data Transmission in P1394b
FIG. 4 is a representation of the prior art data packet structure according to the P1394b standard. As is known by those of ordinary skill in the art, P1394b utilizes a packet structure that is scaled to speed unlike the fixed timer system utilized in Legacy standard. Specifically, the packet structure in P1394b is based upon symbols, rather than the fixed intervals found in the Legacy standard.
In FIG. 4, a typical prior art packet 400 of P1394b data is shown. As is known by those of ordinary skill in the art, a data packet begins in P1394b with the transmission of at least two packet starting symbols. In this example, a Speed Code symbol Sb1 and a Data Prefix symbol DP1 are shown as the packet starting symbols. Then, P1394b data bytes B1 through Bn are transmitted. Data bytes B1 through Bn may be referred to herein as the payload. Finally, the transmission of data is terminated by transmitting DE symbols DE1 and DE2.
Compatibility Problems Between P1394b and Legacy Nodes and Clouds
FIG. 5 shows a data communications system comprising both P1394b and Legacy devices. This type of system will be referred to herein as a “hybrid” system. FIG. 5 shows a Legacy node #0 connected to Legacy node #4, forming what is known as a “Legacy cloud” of Legacy nodes.
Legacy node #4 is connected to border node #3. Border node #3 is connected to B PHYs #1 and #2, forming what is known as a “Beta cloud” of border and B PHYs.
As mentioned above, the data prefix in IEEE 1394-1995 is at least 140 ns in length, and the data end is 240 ns. Therefore, the smallest Legacy packet, including the mandatory idle time that must follow, is at least 400 ns in length. As is appreciated by those of ordinary skill in the art, the problem of compatibility arises because this amount of time can be greater than an entire data packet in P1394b.
Therefore, in order for Beta devices to communicate with each other, any Legacy devices must at the same time be protected from receiving Beta packets. As can be seen from FIG. 5, this protection must be accomplished by the border node, since it is the node connecting the Legacy and Beta clouds together.
Currently, there is no provision for allowing a Beta cloud to communicate within itself while connected to a Legacy cloud.
Therefore, there is a need for a protocol that allows a border node to manage Legacy nodes while communication with a Beta cloud occurs and does not affect the delivery of Alpha format or Legacy packets.
Finally, there is a corollary need for a protocol to speed filter slower beta devices from higher speed packets within a beta cloud.